Hitherto, conjugated fibers, obtained by conjugating two or more kinds of acrylic polymer in a bimetal formation (i.e. with the polymers appearing as sectors in a cross-section through the conjugated fiber) or as a sheath-core formation through a conjugation spinneret, have unique and excellent three dimensional crimps have been widely applied to such uses as clothing, wadding for bedding and the like.
However, uneven dyeing and peeling is likely to occur owing to the differences in kind and composition of the polymers. Moreover, it is generally necessary to increase the number of crimps for obtaining conjugated fibers having high bulkiness, but the touch of fibers then tends to become hard because the degree of shrinkage does not increase in proportion to the crimp numbers per length. These are the defects of the products based on conjugated fibers.
Furthermore, the spinneret device becomes more expensive as spinneret structures become more complex in spinning technology and it is also especially difficult to produce conjugated fibers of finer denier. Moreover, there has been the problem that only inferior touch far below the touch of wool could be obtained and so on because the conjugated state of the fibers obtained is too uniform. Against these conventional bimetal type or sheath-core type conjugated fibers, Japanese Laid-Open Patent Applications Nos. 70322/1976 and 75151/1976 proposed a multi-layered conjugated fiber produced by introducing different spinning dopes of acrylic polymers into a static mixer to divide them to form a multi-layered flow and thereafter spinning this flow through a spinneret. It is said that the multi-layered conjugated fibers thereby obtained give spun yarns and their products without occurrence of uneven yarns and with uniform bulkiness.
However, although the multi-layered conjugated fibers thus obtained give improvements in blending and bulkiness to some extent in comparison with the effect of the conventional conjugated fibers, the theoretical number of layers per fiber expressed as the statistical average number of inflow dope layers per filament, (i.e. layers caused to flow into each hole of a spinneret are both low, namely 1.0-2.0 and 0.05-0.5 in Japanese Laid-Open Patent Applications No. 70322/1976 and 75151/1976 respectively, because the unique cross-sectional structures and physical characteristics of the fibers are no longer maintained when division of layers is too high in the static mixer. Therefore, as shown in FIG. 2, fibers consisting of a single component polymer, namely only one component polymer of the conjugated polymers, become included in large quantities in the conjugated fibers. As a result, because of insufficient multi-layered conjugation of two or more polymer components, there are defects that not only can the required shrinkage characteristics hardly be obtained, but the shrinkage characteristics fluctuate remarkably. This tendency becomes more noticeable with increasing molar ratio difference between copolymer compositions. Moreover, as described above, when the content of single fibers consisting of only one component polymer of the conjugated polymers becomes higher, unevenness of shrinkage characteristics and uneven dyeing occur as a matter of course. Further problems still remain in the conventional multi-layered conjugated fibers of the prior art.
The theoretical number of layers per fiber can be expressed as the statistical average number of inflow dope layers caused to flow into each hole of a spinneret. This is a theoretical value of the number of layers being theoretically brought into a single fiber in the region of a perfect laminar flow, and can be calculated by the following equation: ##EQU1## (where K is a constant determined by the outer shape of the spinneret plate and the value of K is 1 for a rectangular shape and 1.1 for a circular shape).
On the other hand, as, for example, Japanese Patent Publication No. 32859/1979 shows, a modified cross sectional acrylic fiber having a shrinking percentage of 15-25% in drying and successive processes can be prepared by spinning an acrylonitrile polymer comprising 95 mole % or more of acrylonitrile and 0.7-2.0 mole % of vinyl monomer containing sulfonic acid groups through a spinneret whose cross-section has three or more protruding portions of an acute or obtuse angle under a spinning draft of 0.9-1.5.
However, the conventional modified cross-sectional fibers display various unsolved problems described below, which have not yet been solved. Thus, their mechanical properties, especially tensile strength and elongation and knot strength, are lower than those of ordinary acrylic fibers and flies and fluffs therefore very often occur in the spinning process. There is moreover another problem peculiar to the modified cross-sectional fibers that if the composition is modified to try to solve this problem, color deepness after dyeing becomes insufficient owing to insufficiency of denseness and luster. Moreover, there is the further problem that bulkiness of the modified cross-sectional fibers cannot reach a sufficiently satisfactory level.
As an example of an attempt to give water absorbent property to acrylic fibers, Japanese Laid-Open Patent Application No. 139510/1982 discloses that water absorbent property can be given to acrylic fibers by treating acrylic fibers containing a carboxylic acid component with boiling aqueous alkali solution.
However, there are problems in the conventional water absorbent acrylic fibers, in that the mechanical strength after giving water absorbent property (ordinary alkali treatment) is lower in comparison with that of the ordinary acrylic fibers, the dyeing property is insufficient, stickiness to the touch after water absorption is large, and moreover, because it is difficult to give appropriate crimps to water absorbent acrylic fibers, it is therefore difficult to obtain a bulky touch.
Pilling-resistant acrylic fibers are well known. However, it has been difficult to obtain pilling-resistant acrylic fibers having good balance of dyeing property, bulkiness and knot strength after treating with boiling water.